A plasma is a gaseous collection of ions, neutral atoms or molecules, and free electrons. Plasmas are electrically conductive because the unbound charged particles couple easily to electromagnetic fields. While the definition of the term “plasma” can vary, it usually includes some element of “collective” behavior, meaning that any one charged particle can interact with a large number of other charged particles in the plasma.
There are a number of applications which require miniaturized plasma sources. These applications include bio-sterilization, small scale materials processing and microchemical analysis systems.
Hopwood et al., U.S. Pat. No. 6,917,165, describes the use of a microstrip resonator at microwave frequencies for producing “non-thermal” plasmas at a gap in the same plane as the resonator. FIG. 1 illustrates a device 100 disclosed by of Hopwood. Device 100 includes a substrate 10 of dielectric material, a stripline 12 provided on substrate 10 and connected at one end to a coaxial connector 14 and at the other end to a high Q split-ring resonator (hereinafter “resonant ring”) 16 having a discharge gap 18 in the plane of substrate 10. A ground plane 20 is provided on the opposite side of the substrate 10 from resonant ring 16.
Stripline 12 is one-quarter wavelength (λ/4) in length at the operating frequency and serves as a quarter wave transformer to match the impedance of resonant ring 16 to the impedance of a power supply which energizes the generator. The impedance is typically 50 ohms. The circumference of resonant ring 16 is one-half wavelength (λ/2) at the operating frequency. The angle between discharge gap 18 and the centerline of resonant ring 16 is such that the impedance measured at the power input at connector 14 is matched to that of the power supply. Voltages at the ends of resonant ring 16 on either side of discharge gap 18 are 180 degrees out of phase with each other.
The electric field at discharge gap 18 is further enhanced by the Q of resonant ring 16, where Q is the quality factor of the resonator [Q=2π(energy stored/energy dissipated)], and the small dimension (in general less than 50 μm) of discharge gap 18. Accordingly, high electric fields are available across discharge gap 18. If a gas-confining structure is provided at an area spanning discharge gap 18, the high voltage across discharge gap 18 can strike the gas to form a microplasma discharge.
FIG. 2 illustrates another device 200 disclosed by Hopwood. Device 200 includes a high Q split-ring resonator (hereinafter “resonant ring”) 40 with a discharge gap 42 between confronting ends of the stripline, and an input connector 41 on the stripline, disposed on a first surface of a substrate 46. A ground plane (not shown) is provided on the opposite side of substrate 46. Discharge gap 42 is typically 50 μm. Connector 44 and discharge gap 42 are in positions on resonant ring 40 to provide an impedance matched to that of the power supply, typically 50 ohms. Compared to device 100, the λ/4 transmission line is eliminated and impedance matching of resonant ring 40 is accomplished by the dimensions of resonant ring 40 and the position of input connector 44 and discharge gap 42 on resonant ring 40.
Dutton et al., U.S. Patent Application Publication 2007/0170995, describes a plasma generator with a split-ring resonator and a gas flow element configured to flow gas through a discharge gap in the split-ring resonator. U.S. Patent Application Publication 2007/0170995 is incorporated herein by reference for all purposes as if fully set forth herein.
FIG. 3 illustrates a split-ring resonator device 300 disclosed by Dutton. The left hand side of FIG. 3 shows a top view of device 300, and the right hand side of FIG. 3 shows a cross-sectional view through line I-I′ in the top view of device 300. Device 300 includes a planar substrate 310 (planar in the X/Y plane) of dielectric material, a stripline transmission line 312 provided on a first (“top”) surface of substrate 310 and having a first end 314 and an opposite second end connected to a split-ring resonator (hereinafter “resonant ring”) 316 having a discharge gap 318. A ground plane 320 is provided on a second (“bottom”) side of substrate 310 opposite from the first side that includes resonant ring 316. Device 300 includes a gas flow element 328 configured in operation (e.g., during plasma generation) to flow a stream of gas through discharge gap 318. In the embodiment shown in FIG. 3, the gas flow element 328 extends through substrate 310 and is configured to flow gas substantially orthogonally to the X/Y plane (i.e., substantially in the Z direction).
FIG. 4 shows a plasma generation device 400 disclosed by Dutton which includes split-ring resonator device 300, an RF/microwave connector 440, a power source 460, a gas supply 470, and a gas feed connector 472. The left hand side of FIG. 4 shows a top view of plasma generation device 400, and the right hand side of FIG. 4 shows a cross-sectional view through line I-I′ in the top view of plasma generation device 400. In operation, plasma generation device 400 may produce a plasma jet 50 that emanates from gas flow element 328.
Although the devices described above can usefully generate plasma, there is a continuing interest in developing devices which can operate more efficiently, generate greater plasma densities, or are just generally better suited for particular applications.